This invention relates to control systems and methods for controlling the thrust of a multi-spool turbofan engine.
In controlling the thrust of a multi-spool turbofan engine and especially in causing selected changes in the thrust output of such an engine, it is important that a thrust control system provide both accuracy and stability. Providing acceptable accuracy and stability in such controls has been a problem of long standing in the art.
The accuracy and stability of thrust control is determined in large part by feedback signals from the engine and by the treatment of these signals by the thrust control system. One approach to stabilizing the operation of a two spool turbofan engine has been to monitor the difference between the actual rotational speed of the high-speed spool and a selected rotational speed for the high-speed spool and then control fuel flow to the engine in response to this difference. However, it is known that certain engine pressure-related parameters are more directly related to, and are much better indicators of, engine thrust than is the speed of the high-speed spool; and it is therefore more advantageous, in terms of accuracy, to utilize engine parameters such as engine pressure ratio (EPR) or integrated engine pressure ratio (IEPR) to control engine operation.
Unfortunately, in a multi-spool turbofan engine there is more inertia associated with engine pressure parameters such as EPR and IEPR than there is with the speed of the high-speed spool, so that although accuracy is improved with the use of parameters such as EPR and IEPR there is also some loss of stability. In addition, the pressure signals used to determine values of the relevant pressure-related parameters usually include an element of noise which must be filtered out, and the filtering step further delays response time and has a negative impact on thrust control stability. Thus, it has been necessary in the past to sacrifice one of either accuracy or stability in order to improve the other, and engine thrust control systems have not afforded the combination of accuracy and stability to the degree desired in the environment of modern jet aircraft.